Asterisks and underscores don't interfere with each other anymore. In particular, your *A_B* will look as expected.

Intra-word emphasis is really disabled. Asterisks and underscores are not supposed to have any effect when they appear inside words, but in cases like foo__bar__baz and 2**5 + 4**3 they still did.

***text*** now renders as <strong><em>text</em></strong>, not incorrectly nested as <em><strong>text</em></strong>. This wasn't usually a user-visible problem (the browsers handle it fine), but could cause some weird-looking revision diffs, for example.

I have tested this change on 50,000 Meta posts (where every Markdown edge case that exists is exploited somewhere, so it's a good test bed). The only place where this made a difference that wasn't clearly positive was this question, where the italicized footnote was created with **We would...*. The fact that this used to work was more an accident.

All other locations where I found a difference caused by this change are a definite improvement.